The opaque PostUserEvent IDs are actually pointers

...so declare them as such.  This avoids the recurring mistake of storing such
IDs as sal_uInt32, truncating in 64 bit environments, causing RemoveUserEvent to
potentially not remove the event, it thus firing "too late" and probably causing
a crash.

While at it, consolidate the trivially unnecessary overloads of both
Application::PostUserEvent and Window::PostUserEvent.  And in each of them, it
looks like deleting the mpLink member was missing from the failure branch.

Change-Id: Iab13afbb06e12ac15dec6a6b5b85a7e402a3c654